Run flat pneumatic tire with shoulder cushion rubber layer loss tangent less than carcass coating rubber loss tangent

ABSTRACT

There is provided a pneumatic tire having an aspect ratio of not less than 60 and considerably improving run flat durability as compared with the conventional type of the run flat tire, wherein at least one cushion rubber layer having a loss tangent lower than a loss tangent (tan δ) of a coating rubber for cords of a carcass ply is arranged between an end portion of a belt and a reinforcing rubber layer in a shoulder region and between mutually adjacent tire constitutional members. And also, when the carcass is comprised of plural plies, at least one ply is separated off in a zone beneath the belt, whereby the ride comfort against vibrations can be more improved.

This is a division of application Ser. No. 10/463,479 filed Jun. 18,2003, now U.S. Pat. No. 6,889,736 B1 issued May 10, 2005 which is adivision of application Ser. No. 09/622,445 filed Aug. 17, 2000 (nowU.S. Pat. No. 6,619,354 B1 issued Sep. 16, 2003), which was a NationalStage Application filed under the provisions of 35 U.S.C. 371 based onInternational Application No. PCT/JP99/06023 filed Oct. 29, 1999. Theentire disclosures of the prior applications are hereby incorporated byreference.

TECHNICAL FIELD

This invention relates to a pneumatic tire, and more particularly to aradial tire of so-called run flat type capable of running over a givendistance at a state that an internal pressure becomes zero or isrendered into a slight pressure due to puncture or the like. Especially,it relates to a pneumatic tire having an excellent durability of runflat state (running at puncture state) for use in passenger cars, lighttrucks and small size trucks wherein a ratio of section height tosection width (aspect ratio) is as relatively large as not less than 60.

BACKGROUND ART

Radial tires of run flat type (hereinafter referred to as a run flattire) are mainly used in vehicles wherein a load applied to the tire isrelatively small such as passenger cars, light trucks, small size trucksand the like. In the run flat tire, it is required that when the tire isrendered into a flat state through puncture during not only the runningon general-purpose roads but also the high-speed running on expressways, it is possible to safely and surely run the tire up to a sitecapable of conducting tire exchange over a given distance, for example,not less than 300 km without damaging the steering stability of thevehicle, particularly passenger car and causing the separation off froma rim or the breakage of the tire.

For this end, run flat tires having various structures are proposedsometimes in a combination with a fully worked-out rim to be used. Theseproposed tires are roughly divided into super low section profile tireshaving an aspect ratio of less than 60 and tires having a relativelyhigh section height corresponding to an aspect ratio of not less than60.

The run flat tires mostly and practically used in markets among theabove super low section profile tires are disclosed, for example, inJP-B-45-40483, JP-B-50-12921, JP-A-49, 70303, JP-A-49-116702,JP-A-50-59902, JP-A-50-60905, JP-A-50-60906, JP-A-50-60907,JP-A-50-78003, JP-A-50-111704, JP-A-50-121902, JP-A-50-138502,JP-A-51-20301, JP-A-51-64203 and JP-A-51-69804.

Giving an example of the tires disclosed in the above publications, asshown in FIG. 7, a tire 20 has a structure that a pair of thickreinforcing rubber layers 9 having a crescent shape at section arearranged at an inner face side of an innermost carcass ply 6-1 in aradial carcass 6 extending from a bead portion 2 through a sidewallportion 3 to a tread portion 4. However, this type of the tire isunavoidable in a point that the cost becomes higher and is frequentlymounted onto an expensive grade of a vehicle supposing the high-speedrunning such as sport car, sport-type car or the like.

In the tire 20 having the thick reinforcing rubber layer 9, in order tomitigate the degree of crushed deformation during the running at runflat state under loading (hereinafter referred to as run flat running orrunning under low internal pressure) as far as possible, the radialcarcass 6 has a two or more ply structure comprising a turnup ply 6-1wound around a bead core 5 from an inside of the tire toward an outsidethereof and a down ply 6-2 enveloping the turnup ply 6-1 from an outsidethereof, and a hard stiffener rubber 8 extending from an outerperipheral face of the bead core 5 near to a position of a tire maximumwidth is disposed between the turnup ply 6-1 and the down ply 6-2, and arubberized Kevlar cord or steel cord layer (a layer called as an insertply) is arranged so as to extend from the bead portion 2 to the sidewallportion 3, if necessary.

On the other hand, the tires having the relatively high section heightare frequently used in not only high-grade imported passenger cars andhigh-grade domestic passenger cars having a relatively largedisplacement but also light trucks and small size trucks. This type ofthe tire is possible to run at run flat state in cooperation with a rimfitted thereto, wherein a type of incorporating a protection member(core) for pushing the bead portion toward a rim flange into the rim isthe main current as disclosed in JP-U-56-143102, JP-Y-4-11842,JP-U-2-64405, JP-U-2-64406, JP-A-5-104915, JP-A-6-48125 andJP-A-6-270617.

In the tire having the structure shown in FIG. 7, it is proposed andpracticed to increase the thickness or height of the stiffener rubber 8and the reinforcing rubber layer 9, or largely increase the hardness andmodulus of rubbers 8, 9 themselves for improving the durability in therun flat running. These improving means are proposed under an intentionthat region a in the vicinity of the bead portion 2 and region β of thesidewall portion 3 being trouble region in the run flat running as shownin FIG. 8 are reinforced and at the same time the reinforcing balance ofboth regions α, β is optimized to reduce strain at the region a andstrain at the region β together.

However, as the reinforcing degree of each of the stiffener rubber 8 andthe reinforcing rubber layer 9 increases, the occurrence of troubles inthe region a and the occurrence of troubles in the region β as a tiretrouble in the run flat running can be avoided, while the trouble sitemerely and rapidly shifts to a region γ of a shoulder portion includingan end portion of a belt 7 and an end portion of the reinforcing rubberlayer 9 as shown in FIG. 7. Finally, the thus improved conventional runflat tire can not attain the run flat runnable distance of not less than300 km and support the high-speed running on the expressway as user'sdemand. Therefore, it is still desired to develop tires having a runflat durability capable of running over not less than 300 km andsupporting the high-speed running on the expressway.

And also, the means of incorporating the core into the rim has a problemfirstly in the rim assembling property because the assembling of thetire onto a wheel is not easy. Further, the big rise of weight in thetire and in the tire-wheel assembly is unavoidable and also unspringweight of the vehicle is largely increased to considerably damage theride comfort of the vehicle against vibrations. That is, there is aproblem that these inconveniences become unsuitable in not only the highgrade passenger cars but also the light tuck and small-size truck.

It is, therefore, an object of the invention to provide a pneumatic tireparticularly having an aspect ratio of not less than 60 which can hold agood rim assembling property without causing weight increase in tire-rimassembly and remarkable cost rise, guarantee the safe high-speed runningof a vehicle such as passenger car, light truck, small size truck or thelike even if rapid air leakage is caused by puncture or the like, anddevelop the performance of preventing separation off of the tire fromthe rim during the run flat running over not less than 300 km and thedurability performance.

It is another object of the invention to provide a pneumatic tire havingan aspect ratio of not less than 60 and an excellent ride comfortagainst vibrations during the run flat running, particularly a run flattire for passenger car.

DISCLOSURE OF INVENTION

In order to achieve the above objects, the invention lies in a pneumatictire comprising a carcass of one or more rubberized plies of radiallyarranged cords toroidally extending between a pair of bead coresembedded in respective bead portions and reinforcing a pair of sidewallportions and a tread portion, a belt of two or more steel cord crosslayers disposed on an outer periphery of the carcass to reinforce thetread portion, and a reinforcing rubber layer arranged in at least apart of a zone ranging from a position near to the bead portion throughthe sidewall portion to a shoulder region of the tread portion,characterized in that the belt has end portions at both shoulder regionsof the tread portion;

-   -   at least one cushion rubber layer is provided between the end        portion of the belt and the reinforcing rubber layer in the        shoulder region and between mutually adjacent tire        constitutional members; and    -   the cushion rubber layer has a loss tangent lower than a loss        tangent (tan δ) of rubber for coating cords of the carcass ply.

The term “shoulder region of tread portion” used herein is defined by aregion enclosed between a vertical line drawn to an inner face of aninnermost carcass ply through a ground contact end and a vertical linedrawn to the inner face of the innermost carcass ply through a positionseparated by ⅛ width from the ground contact end toward a central sideof a ground contact area when a ground contact width of the treadportion is divided into eight equal parts at a section of the tire whena tire-rim assembly formed by assembling the tire onto an approved rimis inflated under a slight pressure corresponding to 10% of a maximumair pressure (defined according to JATMA, TRA or ETORTO standard of1998). Moreover, when the tire has a round shoulder, the ground contactend is an intersect between extension lines of two curved lines orbetween extension lines of one curved line and one straight lineconnecting to each end of an arc forming the round.

And also, the term “loss tangent (tan δ)” used herein is a valuemeasured at a testing temperature of 25° C. according to (1) “Casethrough loading waveform, deflection waveform” among non-resonantmethods described in “Testing methods for dynamic properties of curedrubber” of JIS K 6394-1995 when a kind of deformation is tension.

In a preferable embodiment of the invention, the cushion rubber layer isarranged between mutually adjacent end portions of the two steel cordcross layers constituting the belt, between the outermost carcass plyand an end portion of a steel cord layer nearest to the ply, betweenadjacent carcass plies in the carcass comprised of two or more plies, orbetween the innermost carcass ply and the reinforcing rubber layer.

In case of being arranged between the end portions of the steel cordcross layers, the cushion rubber layer has a width within a range of10˜30 mm and is favorable to dividedly arrange the cushion rubber layerat an equal width on both sides with respect to a vertical line VL₁drawn to the outermost carcass ply through an end of a narrow-widthsteel cord layer. This is a case that the mutually adjacent two steelcord cross layers have different widths. If the widths of the crosslayers are the same, the vertical line VL₁ may be a vertical linepassing through any end of the steel cord layers.

In this case, it is favorable that a distance d₁ between adjacent steelcords at the ends of the steel cord cross layers as measured on thevertical line VL₁ is within a range of 0.5˜2.0 mm through the cushionrubber layer.

In case of being arranged between the outermost carcass ply and the endportion of the steel cord layer, the cushion rubber layer has a width of10=40 mm and is favorable to dividedly arrange the cushion rubber layerat an equal width on both sides with respect to a vertical line VL₂drawn to the inner face of the innermost carcass ply through an end ofan innermost steel cord layer constituting the belt.

In this case, it is favorable that a distance d₂ between cord of theoutermost carcass ply and steel cord of the belt layer nearest to such aply as measured on the vertical line VL₂ is within a range of 0.5˜6.0 mmthrough the cushion rubber layer.

In case of being arranged between the carcass plies, the cushion rubberlayer has a width of 10˜30 mm and is favorable to dividedly arrange thecushion rubber layer at an equal width on both sides with respect to thevertical line VL₂.

In this case, it is favorable that a distance d₃ between cords of theadjacent carcass plies as measured on the vertical line VL₂ is within arange of 0.5˜2.0 mm through the cushion rubber layer.

In case of being arranged between the innermost carcass ply and thereinforcing rubber layer, the cushion rubber layer has a width of 10˜40mm and is favorable to dividedly arrange the cushion rubber layer at anequal width on both sides with respect to the vertical line VL₂.

In this case, it is favorable that a distance d₄ from the cord of theinnermost carcass ply to the reinforcing rubber layer as measured on thevertical line VL₂ is within a range of 0.5˜3.0 mm through the cushionrubber layer.

Moreover, it is preferable to arrange the cushion rubber layer in atleast two arrangement positions selected from the above four arrangementpositions.

The loss tangent of the cushion rubber layer is within a range of0.02˜0.10 under test conditions that a temperature is 25° C., an initialtensile load is 160 gf, a dynamic strain is 1.0% and a frequency is 52Hz. Moreover, the method of measuring the loss tangent is as previouslymentioned. However, dimensions of a rubber sample applied to the test ofloss tangent are 2 mm in thickness, 5 mm in width and 20 mm in length.

In another preferable embodiment of the invention, the cord of thecarcass is an organic fiber cord. When the carcass is comprised of twoor more plies, at least one ply contains cords of an organic fiberselected from rayon fiber, aromatic polyamide fiber, aliphatic polyamidefiber having a melting point of not lower than 250° C. as measuredthrough differential scanning calorimetry (DSC) and polyester fiber.

And also, in the carcass comprised of 2 or more plies, at least one plyis favorable to be a split ply separated off in a zone beneath the belt.The split ply is the turnup ply or the down ply and is favorable to havea split-off width corresponding to at least 20% of a belt width.Furthermore, at least one ply of the split plies is favorable to containcords of an organic fiber selected from rayon fiber, aromatic polyamidefiber, aliphatic polyamide fiber having a melting point of not lowerthan 250° C. as measured through DSC and polyester fiber.

As the aliphatic polyamide, nylon-66 or nylon-46 is preferable. As thepolyester, polyethylene terephthalate (PET) andpolyethylene-2,6-naphthalate (PEN) are preferable.

Further, the invention lies in a pneumatic tire for passenger carcomprising a pair of ring-shaped bead cores, a carcass comprised of atleast two rubberized plies of radially arranged cords toroidallyextending between the bead cores to reinforce a pair of sidewallportions and a tread portion, at least one ply of which plies containingcords of an organic fiber selected from rayon fiber, aromatic polyamidefiber, aliphatic polyamide fiber having a melting point of not lowerthan 250° C. as measured through DSC and polyester fiber, a beltarranged on an outer periphery of the carcass to reinforce the treadportion and comprised of two or more steel cord cross layers, and areinforcing rubber layer arranged in at least a part of a zone rangingfrom a position near to the bead portion through the sidewall portion toa shoulder region of the tread portion, characterized in that at leastone ply of the carcass is a split ply separated off in a zone beneaththe belt.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatically left-half section view of a firstembodiment of the pneumatic tire according to the invention.

FIG. 2 is a diagrammatically left-half section view of a secondembodiment of the pneumatic tire according to the invention.

FIG. 3 is a diagrammatically enlarged section view of a main part of thetire shown in FIG. 1.

FIG. 4 is a diagrammatically enlarged section view of a main part of thetire shown in FIG. 2.

FIG. 5 is a diagrammatically enlarged section view of a main part in athird embodiment of the pneumatic tire according to the invention.

FIG. 6 is a diagrammatically enlarged section view of a main part in afourth embodiment of the pneumatic tire according to the invention.

FIG. 7 is a diagrammatically left-half section view of the conventionaltire showing trouble sites together.

FIG. 8 is a diagrammatically right-half section view of the conventionaltire just under loading during the run flat running.

FIG. 9 is a graph showing a relation between run flat durability and tanδ of a cushion rubber layer.

FIG. 10 is a graph showing a relation between run flat durability and awidth of a cushion rubber layer.

FIG. 11 is a graph showing a relation between run flat durability and awidth of another cushion rubber layer.

FIG. 12 is a graph showing a relation between run flat durability and awidth of the other cushion rubber layer.

FIG. 13 is a graph showing a relation between run flat durability and awidth of a still further cushion rubber layer.

FIG. 14 is graph showing a relation between run flat durability and adistance between cords.

FIG. 15 is graph showing a relation between run flat durability andanother distance between cords.

FIG. 16 is graph showing a relation between run flat durability and theother distance between cords.

FIG. 17 is a graph showing a relation between run flat durability and adistance from a carcass ply cord to an inner face of a cushion rubberlayer.

FIG. 18 is a graph showing a relation between run flat durability and a50% modulus ratio of a cushion rubber layer to a reinforcing rubberlayer.

FIG. 19 is a diagrammatically left-half section view of a fifthembodiment of the pneumatic tire according to the invention.

FIG. 20 is a diagrammatically left-half section view of a sixthembodiment of the pneumatic tire according to the invention.

FIG. 21 is a diagrammatically left-half section view of a seventhembodiment of the pneumatic tire according to the invention.

FIG. 22 is a diagrammatically left-half section view of an eighthembodiment of the pneumatic tire according to the invention.

FIG. 23 is a diagrammatically left-half section view of a ninthembodiment of the pneumatic tire according to the invention.

FIG. 24 is a diagrammatically left-half section view of a tenthembodiment of the pneumatic tire according to the invention.

FIG. 25 is a diagrammatically left-half section view of an eleventhembodiment of the pneumatic tire according to the invention.

FIG. 26 is a diagrammatically left-half section view of a twelfthembodiment of the pneumatic tire according to the invention.

FIG. 27 is a diagrammatically left-half section view of a thirteenthembodiment of the pneumatic tire according to the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

A pneumatic tire 1 for passenger car shown in each of FIG. 1 and FIG. 2comprises a pair of bead portions 2 (only one is shown), a pair ofsidewall portions 3 (only one is shown), a tread portion 4 connecting toboth sidewall portions 3, and a carcass 6 extending between a pair ofbead cores 5 embedded in the respective bead portions 5 to reinforce theabove portions 2, 3, 4 and comprised of one or more rubberized plies ofradially arranged cords, two plies in the illustrated embodiment.

The carcass 6 of the illustrated embodiment comprises a turnup ply 6-1having a turnup portion wound around the bead core 5 from an inside ofthe tire 1 toward an outside thereof, and a down ply 6-2 enclosing amain body and turnup portion of the turnup ply 6-1 from the outsidethereof between the mutual bead cores 5 and having a terminal end nearto the bead core 5. When the carcass 6 is one ply, it is the turnup ply6-1. In the carcass 6 of the illustrated embodiment comprised of 2plies, the down ply 6-2 is an outermost carcass ply. As a cord in eachply 6-1, 6-2 of the carcass 6, it is possible to use either of anorganic fiber cord such as nylon cord, polyester cord, rayon cord or thelike and steel cord.

On an outer periphery of the carcass 6 is provided a belt 7 reinforcingthe tread portion 4. The belt 7 is comprised of two or more layers, twosteel cord cross layers 7-1, 7-2 in the illustrated embodiment. The cordcross layers have a structure that cords of the adjacent layers arecrossed with each other with respect to an equatorial plane E of thetire. In the steel cord cross layers 7-1, 7-2 of the illustratedembodiment, a width of the steel cord layer 7-1 adjacent to theoutermost ply or down ply 6-2 of the carcass 6 is wider than a width ofthe outer steel cord layer 7-2.

As shown by dotted lines in FIGS. 1 and 2, the belt 7 of the illustratedembodiment is provided with a helically wound layer 7-3 of an organicfiber cord such as nylon-66 cord or Kevlar cord enclosing the two steelcord cross layers 7-1, 7-2 from the outside thereof, but the helicallywound layer 7-3 is not always required. Therefore, the end portion ofthe belt 7 means end portions of the steel cord cross layershereinafter.

Further, the tire 1 comprises a stiffener rubber 8 taperingly extendingfrom an outer peripheral surface of the bead core 5 toward the treadportion 4. The stiffener rubber 8 is enclosed with the main body andturnup portion of the turnup ply 6-1.

And also, the tire 1 is provided at an inner face side of the turnup ply6-1 as an innermost ply of the carcass 6 with a reinforcing rubber layer9 having a crescent shape at a section thereof inherent to the run flattire. The reinforcing rubber layer 9 is arranged in a region rangingfrom the vicinity of the bead core 5 through the sidewall portion 3 tothe shoulder region of the tread portion 4 so that a central part in theradial direction of the tire is a thick part of 8˜12 mm in maximum gaugeand both end parts in the radial direction are tapered in order tostably support total weight of the vehicle during the running even at aninternal pressure of zero and prevent separation off of the tire 1 fromthe rim used and prevent the breakage of the tire 1 and further hold therunning stability even if rapid puncture is caused during the high-speedrunning at, for example, 80˜120 km/h to make possible the support ofhigh-speed running and the complete running over not less than 300 kmunder run flat condition without causing troubles.

Moreover, the reinforcing rubber layer 9 is made from a rubbercomposition or a fiber-reinforced rubber composition. Further, thereinforcing rubber layer may be divided into multi layers. And also, theshape of the reinforcing rubber layer may be a rubber sheet as it is inaddition to the crescent shape at section. Since the reinforcing rubberlayer is enough to reinforce at least a part of the sidewall portion,the arranging position is not particularly restricted. However, in caseof the reinforcing rubber layer having the crescent shape at section, itis favorable to be arranged along the inner face side of the main bodyof the turnup ply in the sidewall portion, and in case of the sheet-likereinforcing rubber layer, it is favorable to be arranged inside oroutside the main body of the turnup ply in the sidewall portion, or itis favorable to use both the cases together.

Referring to FIGS. 1 and 2, a region enclosed between a vertical lineVL_(E) drawn to an inner face of an innermost carcass ply (turnup ply6-1) through a ground contact end TE and a vertical line VL_(S) drawn tothe inner face of the innermost carcass ply (turnup ply 6-1) through aground contact position S separated by ⅛ width from the ground contactend TE toward a side of the equatorial plane E of the tire when a groundcontact width W of the tread portion 4 is divided into eight equal partsat a section of the tire when a tire-rim assembly formed by assemblingthe tire 1 onto an approved rim (not shown) is inflated under a slightpressure corresponding to 10% of a maximum air pressure calls theshoulder region as previously mentioned.

In this case, both end portions of the steel cord layer forming thesteel cord cross layers of the belt 7 are existent in both the shoulderregions. In the belt 7 of the illustrated embodiment, the end portion ofeach of the steel cord cross layers 7-1, 7-2 is existent in eachshoulder region. The tire 1 is provided with at least one cushion rubberlayer arranged between the end portion of the belt 7 and the reinforcingrubber layer 9 in the shoulder region and between mutually adjacent tireconstitutional members. As the cushion rubber layer arranged between themutually adjacent tire constitutional members, there are four kinds ofcushion rubber layers 11, 12, 13 and 14, which are shown in FIGS. 1˜6every the layer (FIGS. 3 and 4 are partial enlarged views of FIGS. 1 and2, respectively).

In this case, as the end portion of the belt 7 in the shoulder region isadopted an end portion of a narrow-width steel cord layer in the steelcord cross layers 7-1, 7-2, or an end portion of the steel cord layer7-2 located apart from the carcass 6 in the illustrated embodiment.Moreover, this end portion means a portion ranging inward from the edgeof the belt layer by a distance of, for example, 10˜20 mm in thewidthwise direction.

Among the mutually adjacent tire constructional members between the endportion of the belt 7 and the reinforcing rubber layer 9 in the shoulderregion, two constitutional members located at the outermost side in theradial direction are the steel cord cross layers 7-1, 7-2. The tire 1having a first cushion rubber layer 11 arranged between the end portionsof these mutual steel cord layers 7-1, 7-2 is shown in FIG. 1 (FIG. 3).Two constitutional members located at the innermost side in the radialdirection are the innermost ply, the turnup ply 6-1 in the illustratedembodiment and the reinforcing rubber layer 9. The tire 1 having afourth cushion rubber layer 14 arranged between these mutual members isshown in FIG. 2 (FIG. 4). Moreover, when the belt 7 is comprised ofthree or more steel cord cross layers in the tire 1 shown in FIG. 1, atleast one of the first cushion rubber layer 11 is arranged between themutual end portions among these layers.

And also, there are the outermost ply, the down ply 6-2 in theillustrated embodiment and the steel cord layer 7-1 as the mutuallyadjacent tire constructional members. A main part of the tire 1 having asecond cushion rubber layer 12 arranged therebetween is shown in FIG. 5.As the other mutually adjacent tire constitutional members, there arethe turnup ply 6-1 and the down ply 6-2. A main part of the tire 1having a third cushion rubber layer 13 arranged therebetween is shown inFIG. 6. In the tire 1 shown in FIG. 6, when the carcass 6 has pluralturnup plies 6-1, or when the carcass 6 has plural down plies 6-2, atleast one of the third cushion rubber layer 13 is arranged between themutual turnup plies 6-1 or between the mutual down plies 6-2,respectively. Moreover, the cushion rubber layer 11, 12, 13, 14 is madeof rubber having a loss tangent (hereinafter shown as tan δ) smallerthan tan δ of coating rubber for cords of the ply constituting thecarcass 6.

The function and effect by arranging the cushion rubber layer 11, 12,13, 14 will be described below.

FIG. 8 is a diagrammatically right-half section view illustrating abehavior of bending deformation just under loading or in the vicinitythereof during run flat running of the conventional tire 20 assembled ona rim 15 with a flange 15F (FIG. 7). As shown in FIG. 8, the reinforcingrubber layer 9 in the region γ shown in FIG. 7 is subjected tocompression due to the large bending deformation of the sidewall portion3 as a whole, so that an outer portion of the reinforcing rubber layer 9in the radial direction is pushed out toward the equatorial plane E ofthe tire. By such a pushed deformation is applied a force in a directionof arrow toward the side of the equatorial plane E of the tire to theply of the carcass 6 shown by dotted lines.

On the other hand, the steel cord cross layers 7-1, 7-2 of the belt 7having a high stiffness even under an internal pressure of zero stronglycounter to buckling deformation of the tread portion 4, so that the endportion of the belt 7 having free ends intends to move toward theoutside of the tire and hence force in a direction of arrow b oppositeto the direction of arrow a is applied to the ply of the carcass 6. Theforces in the opposed directions of arrows a and b create shearingstrain γ_(P) between the mutually adjacent constitutional membersexisting between the end portion of the belt 7 and the reinforcingrubber layer 9 in the region γ opposite thereto.

As the stiffener rubber 8 and the reinforcing rubber layer 9 are morestrengthened for preventing troubles in the region α and region β (seeFIG. 7), the force in the direction of arrow a and force in thedirection of arrow b more increase, and as a result it could beelucidated that

-   (1) the shearing strain γ_(P) increases;-   (2) the increasing zone of the shearing strain γ_(P) in problem is    within a range from an edge of the steel cord layer 7-1 located    inside the belt 7 in the radial direction to about 5 mm toward the    outside of the tire and to about 10 mm toward the inside of the    tire; and-   (3) the increase of the shearing strain γ_(P) exerts upon-   (i) increase of shearing strain γ_(P1) between mutual end portions    of the belt 7,-   (ii) increase of shearing strain γ_(P2) between the end portion of    the steel cord layer 7-1 in the belt 7 and the ply of the carcass 6,-   (iii) increase of shearing strain γ_(P3) between plies of the    carcass 6 (plural plies) and-   (iv) increase of shearing strain γ_(P4) between the innermost ply of    the carcass 6 and the reinforcing rubber layer 9.

By these large shearing strains γ_(P1), γ_(P2), γ_(P3), γ_(P4) iscreated a big shearing deformation between mutually laminatedconstitutional members arranged between the end portion of the belt 7and the reinforcing rubber layer 9 in the shoulder region. The bigshearing deformation is repeated to cause heat build up in rubber of thelaminated constitutional members and finally the rise of temperature dueto a greater amount of heat build up brings about heat breakage ofrubber in the laminated constitutional members. Especially, it has beenfound out that the big shearing deformation between the mutual plies6-1, 6-2 of the carcass 6 and the heat breakage due to the great amountof heat build up accompanied therewith are serious and they compositelycause the trouble in the region γ.

For this end, at least one of the cushion rubber layer 11, 12, 13, 14 isarranged between the mutually adjacent tire constitutional memberslocated between the end portion of the belt 7 and the reinforcing rubberlayer 9 of the shoulder region in the region γ surrounded by anellipsoidal line in FIG. 8 or in a region of a width Wγ at the troublesite due to the shearing strain γ_(P) to thereby bear the shearingstrain γ_(P) by the cushion rubber layer 11, 12, 13, 14 and hence theshearing strain applied to the constitutional members of the tire 1 canbe mitigated because only the remainder Δγ_(P) deducting the born amountfrom the shearing strain is applied to the constitutional members.

As a result of mitigating the shearing strain γ_(P), the amount of heatbuild up in rubber of each constitutional member at the region γ isreduced and particularly the heat trouble in the plies 6-1, 6-2 of thecarcass 6 hardly occurs. Furthermore, tan δ of the cushion rubber layer11, 12, 13, 14 is smaller than tan δ of coating rubber for the cords inthe plies 6-1, 6-2 of the carcass 6, so that the heat build up of thecushion rubber layer is restricted to a range of small amount and thereis no fear that an evil influence of increasing the amount of the heatbuild up is caused by the arrangement of the cushion rubber layer 11,12, 13, 14.

Since it is possible to avoid the trouble in the region γ by arrangingthe cushion rubber layer 11, 12, 13, 14, the actions of the stiffenerrubber 8 and the reinforcing rubber layer 9 can be more strengthened andhence it is possible to considerably improve the run flat durability ofthe tire 1 by the action of the cushion rubber layer 11, 12, 13, 14 andthe stiffener rubber 8 and the reinforcing rubber layer 9 together.

Although the run flat durability is improved by arranging any one of thecushion rubber layers 11, 12, 13 and 14, it is favorable to arrange twoor more layers in combination in order to largely improve the run flatdurability as compared with the conventional tire 20 (see FIG. 7). Evenwhen two or more of the cushion rubber layers 11, 12, 13 and 14 arearranged, the ratio of weight increase is very small in the run flattire having a tire weight heavier than that of the general-purpose tireand the rise of the coat is slight. Moreover, if the line of the carcass6 is slightly shifted toward the inside of the tire, the tread rubber inthe tread portion can be rendered into the same gauge distribution as inthe conventional tire 20, so that the degradation of wear life of thetread rubber is not brought and the rim assembling is not obstructed.

In the light that the trouble in the region γ is due to the heat buildup of the tire constitutional members, the following experiment iscarried out by using radial tires 1 for passenger car having a tire sizeof 225/60R16 for specifying acceptable range of tan δ in the cushionrubber layer 11, 12, 13, 14 arranged in the shoulder region causing thebig shearing strain γ_(P). As the experimental conditions are aninternal pressure of zero (state of taking out a valve core), a load of570 kgf corresponding to 76% of a maximum load capacity of 750 kg (mass)of the above tire described in JATMA YEAR BOOK (1998) and a speed of 89km/h.

All of widths W₁, W₂, W₃, W₄ of the cushion rubber layers 11, 12, 13, 14(see FIGS. 3˜6) are 30 mm, and all of distances d₁, d₂, d₃, d₄ throughthe cushion rubber layers 11, 12, 13, 14 (see FIGS. 3˜6) are 2.0 mm. Thevalue of tan δ are represented by 4 levels under test conditions that atemperature is 25° C., an initial load is 160 gf, a dynamic strain is1.0% and a frequency is 52 Hz. The control is the conventional run flattire 20 shown in FIG. 7.

The running distance till the occurrence of troubles in the tires 1 and20 is evaluated as the run flat durability. The experimental results areshown in FIG. 9 as a relation between running distance on drum (index)represented by an index on the basis that the conventional tire 20 is100 and tan δ (25° C.) of rubber in the cushion rubber layer 11, 12, 13,14. As seen from FIG. 9, the running distance on drum predominantlyexceeds that of the conventional tire 20 at tan δ of not more than 0.10.And also, when tan b is less than 0.02, the trouble is no longer causedin the region γ, so that the lower limit of tan δ is 0.02 assuming thatthe other performances and durability are equally held as compared withthose of the conventional tire 20. Moreover, mark ● shown in FIG. 9shows trouble in the region γ and mark ▪ shows trouble in a region otherthan the region γ, which are the same in FIGS. 10˜17 as mentioned later.

In order to determine ranges of acceptable widths W₁, W₂, W₃, W₄ of thecushion rubber layers 11, 12, 13, 14 effective for the mitigation ofshearing strains γ_(P1), γ_(P2), γ_(P3), γ_(P4), particularly acceptablewidth W₃ of the cushion rubber layer 13 effective for the mitigation ofshearing strain γ_(P3) between the plies 6-1 and 6-2, experiments arecarried out under the same test conditions as mentioned above. The tan δ(25° C.) of each cushion rubber layer is 0.07, and all of the distancesd₁, d₂, d₃, d₄ are 2.0 mm. The control is the conventional tire 20 ofFIG. 7.

The experimental results evaluating the running distance till theoccurrence of trouble in the tires 1 and 20 as a run flat durability areshown in FIGS. 10˜13 as a relation between running distance on drum(index) represented by an index on the basis that the conventional tire20 is 100 and each of widths W₁, W₂, W₃, W₄ of the cushion rubber layers11, 12, 13, 14, respectively.

It can be seen from FIG. 10 that the width W₁ of the cushion rubberlayer 11 is acceptable within a range of 10˜30 mm, from FIG. 11 that thewidth W₂ of the cushion rubber layer 12 is acceptable within a range of10˜40 mm, from FIG. 12 that the width W₃ of the cushion rubber layer 13is acceptable within a range of 10˜30 mm, and from FIG. 13 that thewidth W₄ of the cushion rubber layer 14 is acceptable within a range of10˜40 mm, respectively.

When the width W₁ of the cushion rubber layer 11 exceeds 30 mm, thewidth W₂ of the cushion rubber layer 12 exceeds 40 mm, the width W₃ ofthe cushion rubber layer 13 exceeds 30 mm, and the width W₄ of thecushion rubber layer 14 exceeds 40 mm, the running distance on drum issaturated and only the tire weight is undesirably increased to transferthe trouble to other region.

In order to determine ranges of acceptable distances d₁, d₂, d₃, d₄ ofthe cushion rubber layers 11, 12, 13, 14 effective for the mitigation ofshearing strains γ_(P1), γ_(P2), γ_(P3), γ_(P4), particularly acceptabledistance d₃ of the cushion rubber layer effective for the mitigation ofshearing strain γ_(P3) between the plies 6-1 and 6-2, experiments arecarried out every the cushion rubber layer under the same testconditions as mentioned above. The tan δ (25° C.) in each cushion rubberlayer is 0.07, and all of widths W₁, W₂, W₃, W₄ are 30 mm. The controlis the conventional tire 20 of FIG. 7.

The experimental results evaluating the running distance till theoccurrence of trouble in the tires 1 and 20 as a run flat durability areshown in FIGS. 14-17 as a relation between running distance on drum(index) represented by an index on the basis that the conventional tire20 is 100 and each of distances d₁, d₂, d₃, d₄ through the cushionrubber layers 11, 12, 13, 14, respectively.

It can be seen from FIG. 14 that the distance d₁ is acceptable within arange of 0.5˜2.0 mm, from FIG. 15 that the distance d₂ is acceptablewithin a range of 0.5˜6.0 mm, from FIG. 16 that the distance d₃ isacceptable within a range of 0.5˜2.0 mm, and from FIG. 17 that thedistance d₄ is acceptable within a range of 0.5˜3.0 mm, respectively.

When the distance d₁ exceeds 2.0 mm, the distance d₂ exceeds 6.0 mm, thedistance d₃ exceeds 2.0 mm, and the distance d₄ exceeds 3.0 mm, therunning distance on drum is saturated and only the tire weight isundesirably increased to transfer the trouble to other region.

Referring to FIG. 3, the first cushion rubber layer 11 having the widthW₁ is arranged on both sides with respect to a vertical line VL₁ drawnto an outer surface of the outermost carcass ply (down ply 6-2) passingthrough an edge of the narrow-width steel cord layer 7-2 among the steelcord cross layers 7-1, 7-2 constituting the belt 7, and referring toFIGS. 4–6, the second to fourth cushion rubber layers 12, 13, 14 havingthe widths W₂, W₃, W₄ are arranged on both sides with respect to avertical line VL₂ drawn to an inner surface of the outermost carcass ply(down ply 6-2) passing through an edge of the wide-width steel cordlayer 7-2 in the belt 7.

It is preferable that the first cushion rubber layer 11 is dividedlyarranged at an equal width of (½)×W₁ on both sides with respect to thevertical line VL₁, and the second cushion rubber layer 12 is dividedlyarranged at an equal width of (½)×W₂ on both sides with respect to thevertical line VL₂, and the third cushion rubber layer 13 is dividedlyarranged at an equal width of (½)×W₃ on both sides with respect to thevertical line VL₂, and the fourth cushion rubber layer 14 is dividedlyarranged at an equal width of (½)×W₄ on both sides with respect to thevertical line VL₂.

In FIG. 3, the distance d₁ is a distance between mutual steel cords Scat the end portions of the steel cord cross layers 7-1, 7-2 through thefirst cushion rubber layer 11 as measured on the vertical line VL₁. InFIG. 4, the distance d₄ is a distance from a cord Tc of the innermostcarcass ply (turnup ply 6-1) to an inner surface 14 is of the fourthcushion rubber layer 14 as measured on the vertical line VL₂. In FIG. 5,the distance d₂ is a distance between the steel cord Sc at the endportion of the steel cord cross layer 7-2 and the cord Tc of theoutermost carcass ply (down ply 6-2) through the second cushion rubberlayer 12 as measured on the vertical line VL₂. In FIG. 6, the distanced₃ is a distance between the cords Tc of the mutually adjacent plies ofthe carcass 6 (turnup ply 6-1, down ply 6-2) through the third cushionrubber layer 13 as measured on the vertical line VL₂.

In addition, it is desirable that 50% modulus of the cushion rubberlayer 11, 12, 13, 14 is lower than 505 modulus of the reinforcing rubberlayer 9. Moreover, an outer coating rubber for the sidewall portion 3can not be applied to the cushion rubber layer 11, 12, 13, 14 becausethis type of the outer coating rubber is essential to have asufficiently excellent resistance to ozone and is obliged to make tan δlarge for satisfying the essential condition.

Furthermore, the cushion rubber layers 11, 12, 13, 14 contribute to theeffect of improving the run flat durability when a percentage ratio(M₅₀C/M₅₀R)×100% of 50% modulus of the cushion rubber layer 11, 12, 13,14 (M₅₀C) to 50% modulus of the reinforcing rubber layer 9 (M₅₀R) is setto an adequate value.

The experiments on 50% modulus ratio (M₅₀C/M₅₀R) are carried out underthe same test conditions as mentioned above except that the cushionrubber layers have tan δ=0.07, widths W₁=W₂=W₃=W₄=30 mm and distancesd₁=d₂=d₃=d₄=2.0 mm. The experimental results evaluating the runningdistance till the occurrence of trouble in the tires 1 and 20 as a runflat durability are shown in FIG. 18 as a relation between runningdistance on drum (index) represented by an index on the basis that theconventional tire 20 is 100 and 50% modulus ratio. As seen from FIG. 18,the value of 50% modulus ratio is adaptable to be not more than 90%,while when the value of 50% modulus ratio is less than 30%, thedifference of stiffness between the cushion rubber layer 11˜14 and thereinforcing rubber layer 9 becomes too large and the trouble istransferred to the cushion rubber layer 11˜14, so that the 50% modulusratio is finally adaptable to be within a range of 30˜90%, desirably60˜82%.

In FIG. 19 is shown a fifth embodiment of the pneumatic tire forpassenger car according to the invention, which has substantially thesame structure as the tire of FIG. 5 except that the carcass 6 iscomprised of two turnup plies 6-1 and down ply 6-2 and the cap ply 7-3is not used. That is, the cushion rubber layer is not shown, but thesecond cushion rubber layer 12 is arranged between the end portion ofthe steel cord layer 7-1 in the belt 7 and the down ply 6-2 as shown inFIG. 5.

In the above pneumatic tire according to the invention, particularlypneumatic tire for passenger car, ones having a further improved ridecomfort against vibrations while holding the excellent run flatdurability are shown in FIGS. 20 to 27. For simplification, however, theillustration of the cushion rubber layer is omitted, but the arrangementof the second cushion rubber layer 12 shown in FIG. 5 is usually used.Moreover, the arrangements of the cushion rubber layers shown in FIGS.3, 4 and 6 may be used.

These tires have the same structure as in FIG. 19 except that theconstruction of the carcass 6 differs as mentioned later. That is, thecarcass 6 is comprised of two or more plies and each of these pliescontains radially arranged and rubberized organic fiber cords therein.At least one ply constituting the carcass 6 contains cords of an organicfiber selected from rayon fiber, aromatic polyamide fiber, aliphaticpolyamide fiber having a melting point of not lower than 250° C. asmeasured through DSC and polyester fiber. By using such fibers isobtained a strong adhesion to rubber member even at a higher temperatureand the peeling at an interface between cord and rubber can beprevented. Especially, the aliphatic polyamide fiber gives a durabilityto heat, light, oxygen or the like, so that it can be used bycompounding with a stabilizer consisting of, for example, a copper saltand an antioxidant. As the aliphatic polyamide fiber, nylon-6,6 andnylon-4,6 are favorable from a viewpoint of an adhesion property at ahigh temperature. And also, when the melting point as measured throughDSC of the cord is lower than 250° C., the holding of the tire shape atthe high temperature becomes difficult and it is apt to cause the fusionand breakage of the cord due to local heat generation and hence it tendsto degrade the durability during the run flat running.

In the carcass comprised of two or more plies, the ride comfort againstvibrations can be more improved by separating off at least one plybeneath the belt.

In general, the reinforcing rubber layer is arranged in the sidewallportion as a structure of the tire capable of running at run flat statefor controlling the bending of the tire at a state that the internalpressure is zero or substantially near to zero. As a result, such a tiretends to damage the ride comfort against vibrations in the usual runningor at the inflation under the normal internal pressure as compared withthe usual tire. For this purpose, when at least one ply is separated offbeneath the belt to from a split ply as mentioned above, the flexibilityis given to a crown portion of the carcass to absorb vibrationstransmitted from a road surface with the crown portion, whereby the ridecomfort against vibrations can be improved.

In a tire 30 shown in FIG. 20, the carcass 6 is comprised of two turnupplies 6-1 each wound around the bead core 5 from an inside of the tire30 toward an outside thereof to form a turnup portion and one down ply6-2 extending between the mutual bead cores 5 to enclose the main bodyand the turnup portion of the turnup ply 6-1 from the outside thereofand having a terminal in the vicinity of the bead core 5, wherein aninnermost turnup ply 6-1 is a split ply formed by separating off theturnup ply in a zone of the crown portion beneath the belt 7 by 40% of awidth of the steel cord layer 7-1 constituting the belt 7 inclusive of acentral line thereof.

In a tire 30 shown in FIG. 21, the carcass 6 is comprised of two turnupplies 6-1 and one down ply 6-1, wherein an outermost turnup ply 6-1 is asplit ply formed by separating off the turnup ply in a zone of the crownportion beneath the belt 7 by 40% of a width of the steel cord layer 7-1constituting the belt 7 inclusive of a central line thereof.

In a tire 30 shown in FIG. 22, the carcass 6 is comprised of two turnupplies 6-1 and one down ply 6-2, wherein the down ply 6-2 is a split plyformed by separating off the ply in a zone of the crown portion beneaththe belt 7 by 40% of a width of the steel cord layer 7-1 constitutingthe belt 7 inclusive of a central line thereof.

In a tire 30 shown in FIG. 23, the carcass 6 is comprised of two turnupplies 6-1 and one down ply 6-2, wherein an innermost turnup ply 6-1 is asplit ply formed by separating off the turnup ply in a zone of the crownportion beneath the belt 7 by 40% of a width of the steel cord layer 7-1constituting the belt 7 inclusive of a central line thereof and anoutermost turnup ply 6-1 is a split ply formed by separating off theturnup ply in a zone of the crown portion beneath the belt 7 by 60% of awidth of the steel cord layer 7-1 constituting the belt 7 inclusive of acentral line thereof.

In a tire 30 shown in FIG. 24, the carcass 6 is comprised of two turnupplies 6-1 and one down ply 6-2, wherein an innermost turnup ply 6-1 is asplit ply formed by separating off the turnup ply in a zone of the crownportion beneath the belt 7 by 40% of a width of the steel cord layer 7-1constituting the belt 7 inclusive of a central line thereof and the downply 6-2 is a split ply formed by separating off the ply in a zone of thecrown portion beneath the belt 7 by 60% of a width of the steel cordlayer 7-1 constituting the belt 7 inclusive of a central line thereof.

In a tire 30 shown in FIG. 25, the carcass 6 is comprised of two turnupplies 6-1 and one down ply 6-2, wherein an outermost turnup ply 6-1 is asplit ply formed by separating off the turnup ply in a zone of the crownportion beneath the belt 7 by 40% of a width of the steel cord layer 7-1constituting the belt 7 inclusive of a central line thereof and the downply 6-2 is a split ply formed by separating off the ply in a zone of thecrown portion beneath the belt 7 by 60% of a width of the steel cordlayer 7-1 constituting the belt 7 inclusive of a central line thereof.

In a tire 30 shown in FIG. 26, the carcass 6 is comprised of one turnupply 6-1 wound around the bead core 5 from an inside of the tire 30toward an outside thereof to form a turnup portion and one down ply 6-2extending between the mutual bead cores 5 to enclose the main body andthe turnup portion of the turnup ply 6-1 from the outside thereof andhaving a terminal in the vicinity of the bead core 5, wherein the turnupply 6-1 is a split ply formed by separating off the turnup ply in a zoneof the crown portion beneath the belt 7 by 40% of a width of the steelcord layer 7-1 constituting the belt 7 inclusive of a central linethereof.

In a tire 30 shown in FIG. 27, the carcass 6 is comprised of one turnupply 6-1 and one down ply 6-1, wherein the down ply 6-2 is a split plyformed by separating off the turnup ply in a zone of the crown portionbeneath the belt 7 by 40% of a width of the steel cord layer 7-1constituting the belt 7 inclusive of a central line thereof.

The split ply as shown in FIGS. 20–27 has a split-off widthcorresponding to at least 20% of a maximum belt width, preferably asplit-off width of 25–70%. In case of plural split plies, at least onesplit ply is favorable to contain cords of an organic fiber selectedfrom rayon fiber, aromatic polyamide fiber, aliphatic polyamide fiberhaving a melting point of not lower than 250° C. as measured through DSCand polyester fiber therein.

In the tires according to the invention, rubber ingredient used in acoating rubber for ply cord of the carcass, the reinforcing rubber layerand the cushion rubber layer is not particularly restricted, but mayinclude, for example, natural rubber (NR), butadiene rubber (BR),styrene-butadiene rubber (SBR) and synthetic isoprene rubber (IR).

The following examples are given in illustration of the invention andare not intended as limitations thereof.

EXAMPLES 1˜39, COMPARATIVE EXAMPLE 1

There are prepared radial tires for passenger car having a tire size of225/60R16 as tires of Examples 1˜39. In this case, the carcass 6 iscomprised of two turnup plies 6-1 and one down ply 6-2 and the otherstructure is according to FIGS. 1, 2 and 3˜6, wherein all plies of thecarcass 6 are rubberized plies of nylon-6,6 cords and the belt 7 iscomprised of two rubberized steel cord cross layers 7-1, 7-2 and one capply 7-3 formed by spirally winding rubberized nylon-6,6 cord. A coatingrubber for cords in each of the plies 6-1, 6-2 of the carcass 6 has tanδ of 0.16, and a coating rubber for cords in the cord cross layers 7-1,7-2 of the belt 7 has tan δ of 0.15.

In order to evaluate the run flat durability of each example tire, thereare provided a conventional tire and a tire of Comparative Example 1. Inthe conventional tire, a rubber layer corresponding to the cushionrubber layer 12 has tan δ of 0.16. In Tables 1 and 2 are shown tan δ ofcushion rubber layer 11, 12, 13, 14, distances d₁, d₂, d₃, d₄ (mm) onvertical line VL₁, VL₂, widths W₁, W₂, W₃, W₄ (mm) and value of 50%modulus ration (M₅₀C/M₅₀R) {represented by M₅₀ ratio (%)} in each ofthese tires.

Each of the tires is assembled onto a recommended rim among approvedrims (according to JATMA standard), and inflated under a sufficient airpressure to render the tire into use state and then the air pressure isreturned to zero. Each tire rendered into run flat state is pushed ontoa drum rotating at a surface speed of 89 km/h under a load of 570 kgfcorresponding to 76% of a maximum load capacity to measure a runningdistance until the occurrence of trouble in the tire (run flatdurability). The measured results are represented by an index on thebasis that the conventional tire is 100 and shown in Tables 1 and 2. Thelarger the index value means the better the run flat durability.

TABLE 1 Distance on vertical line VL₁, VL₂ Width of cushion rubber layer(mm) (mm) M₅₀ ratio Running distance Kind of tire tan δ d₁ d₂ d₃ d₄ W₁W₂ W₃ W₄ (%) on drum (index) Conventional 0.16 — 1.5 — — — 20 — — 75 100Example Comparative 0.16 2.0 2.0 2.0 2.0 30 30 30 30 75 100 ExampleExample 1 0.10 2.0 2.0 2.0 2.0 30 30 30 30 75 115 Example 2 0.07 2.0 2.02.0 2.0 30 30 30 30 75 145 Example 3 0.04 2.0 2.0 2.0 2.0 30 30 30 30 75172 Example 4 0.02 2.0 2.0 2.0 2.0 30 30 30 30 75 180 Example 5 0.07 0.5— — — 30 — — — 75 103 Example 6 0.07 1.2 — — — 30 — — — 75 107 Example 70.07 2.0 — — — 30 — — — 75 110 Example 8 0.07 3.0 — — — 30 — — — 75 110Example 9 0.07 2.0 — — — 11 — — — 75 105 Example 10 0.07 2.0 — — — 25 —— — 75 109 Example 11 0.07 2.0 — — — 44 — — — 75 110 Example 12 0.07 —1.0 — — — 30 — — 75 112 Example 13 0.07 — 1.0 — — — 30 — — 100 102Example 14 0.07 — 1.0 — — — 30 — — 60 134 Example 15 0.07 — 1.0 — — — 30— — 44 135 Example 16 0.07 — 3.0 — — — 30 — — 75 134 Example 17 0.07 —4.3 — — — 30 — — 75 144 Example 18 0.07 — 5.8 — — — 30 — — 75 148Example 19 0.07 — 7.0 — — — 30 — — 75 148 Example 20 0.07 — 2.0 — — — 12— — 75 108 Example 21 0.07 — 2.0 — — — 20 — — 75 128 Example 22 0.07 —2.0 — — — 36 — — 75 144 Example 23 0.07 — 2.0 — — — 48 — — 75 148

TABLE 2 Distance on vertical line VL₁, VL₂ Width of cushion rubber layer(mm) (mm) M₅₀ ratio Running distance Kind of tire tan δ d₁ d₂ d₃ d₄ W₁W₂ W₃ W₄ (%) on drum (index) Example 24 0.07 — — 0.5 — — 30 — 75 105Example 25 0.07 — — 1.2 — — — 30 — 75 112 Example 26 0.07 — — 2.0 — — —30 — 75 116 Example 27 0.07 — — 3.0 — — — 30 — 75 116 Example 28 0.07 —— 2.0 — — — 11 — 75 107 Example 29 0.07 — — 2.0 — — — 25 — 75 115Example 30 0.07 — — 2.0 — — — 44 — 75 116 Example 31 0.07 — — — 0.5 — —— 30 75 105 Example 32 0.07 — — — 1.4 — — — 30 75 128 Example 33 0.07 —— — 2.0 — — — 30 75 137 Example 34 0.07 — — — 3.0 — — — 30 75 140Example 35 0.07 — — — 2.0 — — — 8 75 103 Example 36 0.07 — — — 2.0 — — —20 75 124 Example 37 0.07 — — — 2.0 — — — 34 75 136 Example 38 0.07 — —— 2.0 — — — 44 75 140 Example 39 0.07 0.3 1.2 0.3 1.0 30 40 30 40 75 180

As seen from the results of durability described in Tables 1 and 2, heattrouble of coating rubber for the down ply 6-2 in the region Y is causedin all of the tires of Examples 1˜39, conventional tire and tire ofComparative Example 1. When the durability of each example tire iscompared with that of the conventional tire, the running distance ondrum increases as tan δ of the cushion rubber layer 11, 12, 13, 14becomes smaller, and the running distance on drum increases as the widthW₁, W₂, W₃, W₄ of the cushion rubber layer 11, 12, 13, 14 becomes widerat the same tan δ, and the running distance on drum increases as thevalue of the distance d₁, d₂, d₃, d₄ on the vertical line VL₁, VL₂through the cushion rubber layer 11, 12, 13, 14 (or gauge of cushionrubber layer) becomes larger at the same tan δ and width W₁, W₂, W₃, W₄,which show that they are effective to dispersion and mitigation ofshearing strain γ_(P).

And also, it is apparent that the run flat durability exceeding that ofthe conventional tire can be obtained by using any one of the cushionrubber layers 11, 12, 13 and 14, while the use of two or more cushionrubber layers is more effective. In the example tires having an indexvalue of the durability of more than 140, the trouble changes intocracking of the reinforcing rubber layer 9, which shows a limit of theeffect of improving the run flat durability.

In the tire group of Examples 12˜15 in Table 1, the value {M₅₀ ratio(%)} of 50% modulus ratio (M₅₀C/M₅₀R) is changed and the other structureand dimensions are the same as those in the tire of Example 12. When therunning distance on drum (index) is compared among the tire group ofExamples 12˜15, the run flat durability slightly exceeding that of theconventional tire is indicated even in the tire of Example 13 whereinthe M₅₀ ratio is 100%, while the tire of Example 15 wherein the M₅₀ratio is 44% tends to fail to increase the run flat durability asexpected as compared with the tire of Example 14 wherein the M₅₀ ratiois 60%, so that it is apparent that the restriction of the M₅₀ ratio (%)to an adequate range also contributes to the improvement of the run flatdurability as shown in FIG. 18.

A relation between weight increase and running distance on drum in theexample tires is shown in the following Table 3. The weight of theconventional tire is 15.5 kgf. As seen from Table 3, when the tire ofExample 4 is compared with the tire of Example 39 in the use of allcushion rubber layers 11, 12, 13, 14 from a viewpoint of the increase ofthe weight, the increase of the tire weight can be stopped to minimum byproperly thinning the value of the distance d₁, d₂, d₃, d₄ on thevertical line VL₁, VL₂ (gauge of cushion rubber layer).

TABLE 3 Weight increase Running distance on drum Kind of tire (kgf)(index) Conventional Example 0.00 100 Example 4 0.94 180 Example 7 0.20110 Example 23 0.22 148 Example 26 0.20 116 Example 38 0.33 140 Example39 0.32 180

The compounding recipe of rubber composition used in the cushion rubberlayer of Conventional Example and Example 16 is shown in Table 4. Thecushion rubber layer used in each example is prepared by standardizingthe rubber composition of the conventional example and changing kinds ofrubber ingredient, filler and the like to adjust tan δ and M₅₀.

TABLE 4 part by weight Conventional example Example 16 Natural rubber100 80 BR *1 0 20 Carbon black (HAF) 45.0 0 Carbon black (FEF) 0 45.0Softener *2 10.0 0 Stearic acid 1.0 3.0 Vulcanization accelerator *3 0.71.2 Antioxidant *4 1.0 1.0 Zinc white 3.0 10.0 Sulfur 2.5 3.8 50%modulus (MPa) 1.2 2.5 *1: butadiene rubber, BR01, made by JSRCorporation *2: Koumolex 300, made by Nippon Petroleum Chemical Co.,Ltd. *3: Nocceler NS-F, made by Ohuchi Shinko Kagaku Kogyo Co., Ltd. *4:Ozonone 6C, made by Seiko Kagaku Co., Ltd.

EXAMPLES 40–53, COMPARATIVE EXAMPLES 2–3

In this example, the melting point of fiber for the cord of the carcassply through DSC is a peak temperature of a melting curve measuredthrough DSC made by Dunlop under conditions that a temperature risingrate is 10° C./min and a sample weight is about 5 mg.

The tensile stress of rubber composition used in the reinforcing rubberlayer, coating rubber for the cord of the carcass ply and the like ismeasured according to JIS K6301-1995.

The tensile strength and elongation at break of the cord of the carcassply are measured according to JIS L1017-1983.

The tire performances are measured by the following methods.

(1) Ride Comfort Against Vibrations

A test tire adjusted to an internal pressure of 2.0 kg/cm² is trained ona drum having an outer diameter 2000 mm and provided on one place with asemi-circular iron projection having a radius of 5 mm at a speed of 80km/h under a load of 570 kg for 20 minutes, and thereafter the internalpressure is readjsuted to 2.0 kg/cm² at a state of no load and then thespeed is raised to 20 km/h to adjust the load to 570 kg. Thereafter, thespeed is increased every 5 km/h and an average waveform of load changeto a tire-fixed shaft in the riding over the projection is measured ateach speed, from which p-p value is calculated.

A changing direction of shaft load in the riding over the projection atthe tire-fixing shaft is a forward direction of the tire (longitudinalspring), and so-called longitudinal spring constant becomes maximum at aspeed zone of 30˜40 km/h. Therefore, the p-p value (kg) at such a speedzone is calculated to evaluate the ride comfort against vibrations.

Moreover, the ride comfort against vibrations of the test tire isdetermined according to the following equation 1 and represented by anindex on the basis that Comparative Example 2 or 3 as a control tire is100.Ride comfort of test tire againstvibrations=100+100×{(p−p)c−(p−p)t)}/(p−p)c  (1)wherein {(p−p)c} is a p−p value of the control tire and {(p−p)t} is ap−p value of the test tire.

The larger the index value, the better the ride comfort againstvibrations.

(2) Run Flat Durability

The run flat durability is evaluated by the same method as described inExample 1.

There are first prepared radial tires for passenger car having a tiresize of 225/60R16 and various carcass structures as shown in FIGS. 1 and19–27.

As the cushion rubber layer is used the rubber composition of Example 16shown in Table 4, and compounding recipes of rubber compositions for thereinforcing rubber layer and the coating rubber for the cord of thecarcass ply are shown in Tables 5 and 6.

TABLE 5 Reinforcing rubber layer (part by weight) Natural rubber 30.0Butadiene rubber *1 70.0 Carbon black (FEF) 60.0 Spindle oil 5.0 Zincwhite 3.0 Stearic acid 1.0 Antioxidant *2 2.0 Vulcanization accelerator*3 3.5 Sulfur 5.0 M₅₀: 4.5 MPa *1: BR01, made by JSR Corporation *2:Noclac 6C, made by Ohuchi Shinko Kagaku Kogyo Co., Ltd. *3: Nocceler NS,made by Ohuchi Shinko Kagaku Kogyo Co., Ltd.

TABLE 6 Coating rubber for cord of carcass ply (part by weight) naturalrubber 100.0 Carbon black (HAF) 40.0 Spindle oil 3.0 Zinc white 3.0Stearic acid 1.0 Antioxidant *1 1.0 Vulcanization accelerator *2 0.8Sulfur 2.5 M₅₀: 1.6 MPa *1: Noclac 6C, made by Ohuchi Shinko KagakuKogyo Co., Ltd. *2: Nocceler CZ, made by Ohuchi Shinko Kagaku Kogyo Co.,Ltd.

Tires having the structure shown in FIGS. 24 and 27 are prepared byusing nylon-6 cord (melting point: 218° C.), nylon-6,6 cord (meltingpoint: 258° C.) and nylon-4,6 cord (melting point: 283° C.) as a cordfor the carcass ply and the ride comfort against vibrations and rub flatdurability thereof are evaluated. The results are shown in Table 7.

TABLE 7 Comparative Example Example Comparative Example Example Example2 40 41 Example 3 42 43 Carcass structure FIG. 24 FIG. 24 FIG. 24 FIG.27 FIG. 27 FIG. 27 Kind of cord nylon-6 nylon-4,6 nylon-6,6 nylon-6nylon-4,6 nylon-6,6 Ride comfort 100  99 100 100 100 100 againstvibrations Run flat durability 100 230 205 100 190 150

As seen from Table 7, the run flat durability is largely improved in thetires of the invention as compared with the tires of Comparativeexamples 2 and 3 using nylon-6 having a melting point of not higher than250° C. as a reinforcing cord.

Tires having 10 kinds of carcass structures shown in FIGS. 1 and 19 to27 are prepared by using nylon-4,6 as a cord for the carcass ply and theride comfort against vibrations and run flat durability are evaluated bythe same method as mentioned above to obtain results as shown in Table8. In this case, the tires of Examples 44 and 51 are control.

TABLE 8 Ride comfort Carcass structure against vibrations Run flatdurability Example 44 FIG. 19 100 100 Example 45 FIG. 20 111 100 Example46 FIG. 21 111 100 Example 47 FIG. 22 119 100 Example 48 FIG. 23 117 100Example 49 FIG. 24 118 100 Example 50 FIG. 25 117 100 Example 51 FIG. 1100 100 Example 52 FIG. 26 115 100 Example 53 FIG. 27 113 100

As seen from Table 8, the ride comfort against vibrations can largely beimproved by separating off at least one carcass ply beneath the beltwhile maintaining the run flat durability of the tire.

Tires having 5 kinds of carcass structures shown in FIG. 1, FIG. 19,FIG. 20 and FIG. 27 are prepared by using nylon-6,6 cord, PET cord, PENcord, rayon cord and aromatic polyamide fiber cord (Kevlar) as a cordfor the carcass ply, and the ride comfort against vibrations and runflat durability are evaluated likewise the case of nylon-4,6. Theresults are shown in Tables 9˜13. In this case, the tires of FIG. 1 andFIG. 19 are control.

TABLE 9 Nylon-6,6 cord Carcass structure FIG. 19 FIG. 20 FIG. 22 FIG. 1FIG. 27 Ride comfort 100 111 118 100 114 against vibrations Run flat 100100 100 100 100 durability

TABLE 10 PET cord Carcass structure FIG. 19 FIG. 20 FIG. 22 FIG. 1 FIG.27 Ride comfort 100 108 115 100 111 against vibrations Run flat 100 100100 100 100 durability

TABLE 11 PEN cord Carcass structure FIG. 19 FIG. 20 FIG. 22 FIG. 1 FIG.27 Ride comfort 100 108 114 100 111 against vibrations Run flat 100 100100 100 100 durability

TABLE 12 Rayon cord Carcass structure FIG. 19 FIG. 20 FIG. 22 FIG. 1FIG. 27 Ride comfort 100 110 109 100 114 against vibrations Run flat 100100 100 100 100 durability

TABLE 13 Kevlar cord Carcass structure FIG. 19 FIG. 20 FIG. 22 FIG. 1FIG. 27 Ride comfort 100 106 113 100 111 against durability Run flat 100100 100 100 100 durability

As seen from Tables 9˜13, even when the kind of the cord for the carcasscord is changed, the ride comfort against vibrations can largely beimproved by separating off at least one carcass ply beneath the beltwhile maintaining the run flat durability of the tire.

12 tires having the carcass structures shown in FIGS. 19 and 22 areprepared by using nylon-6,6 cord, PET cord, PEN cord, nylon-4,6 cord,rayon cord and Kevlar cord as a cord for the carcass ply without usingthe cushion rubber layer, and the ride comfort against vibrations isevaluated by the same method as mentioned above to obtain results asshown in Table 14.

TABLE 14 Carcass structure FIG. 19 FIG. 22 Nylon-6,6 cord 100 118 PETcord 100 118 PEN cord 100 115 Nylon-4,6 cord 100 119 Rayon cord 100 115Kevlar cord 100 113

As seen from table 14, the ride comfort against vibrations can largelybe improved by rendering the down ply into split ply irrespectively ofthe kind of the cord.

INDUSTRIAL APPLICABILITY

According to the invention, there can be provided pneumatic tires havingan aspect ratio of not less than 60 which can stop slight cost rise andtire weight increase while well holding both rim assembling property andride comfort against vibrations without using the core and damaging thetire productivity and guarantee safe running of vehicles such aspassenger cars and the like even in the rapid air leakage throughpuncture or the like and enhance the performance of preventingseparation off of the tire from the rim during the run flat running andthe durability performance to a level satisfied by users. And also, theride comfort against vibrations can largely be improved by rendering atleast one carcass ply into split ply as a carcass structure.

1. A pneumatic tire for passenger car comprising: a pair of ring-shapedbead cores embedded in respective bead portions, a carcass comprised ofat least two rubberized plies of radially arranged cords toroidallyextending between the bead cores to reinforce a pair of sidewallportions and a tread portion, at least one ply of which plies containingcords of an organic fiber selected from rayon fiber, aromatic polyamidefiber, aliphatic polyamide fiber having a melting point of not lowerthan 250° C. as measured by differential scanning calorimetry (DSC) andpolyester fiber, a belt arranged on an outer periphery of the carcass toreinforce the tread portion and comprised of two or more steel cordcross layers, and a reinforcing rubber layer arranged in at least a partof a zone ranging from a position near to the bead portion through thesidewall portion to a shoulder region of the tread portion and at aninside of the carcass, wherein at least one ply of the carcass is asplit ply separated off in a zone beneath the belt and wherein thecarcass is comprised of two turnup plies, the split ply is an outermostturnup ply.
 2. A pneumatic tire according to claim 1, wherein the splitpiy has a split-off width corresponding to at least 20% of a width ofthe belt.